In a computer disc drive, data is stored on a computer disc in concentric tracks. In disc drives with relatively high track densities, a servo feedback loop is used to maintain a head over the desired track during read or write operations. This is accomplished utilizing prerecorded servo information either on a dedicated servo disc or on angularly spaced sectors that are interspersed among the data on a disc, During track following, the servo information sensed by the head is demodulated to generate a position error signal (PES), which provides an indication of the position error of the head away from the track center. The PES is then converted into an actuator control signal, which is fed back to control an actuator that positions the head.
As the areal density of magnetic disc drives increases, so does the need for more precise position control when track following, especially in the presence of external shock and vibrations. In order to desensitize disc drives to translational vibrations, a balanced mechanical actuator has been typically used. However, since the actuator must pivot freely to access the data, the effects of rotational vibration about the axis parallel to the pivot axis can be considerable.
To eliminate such errors, a disc drive may introduce cancellation signals into the servo loop that are designed to cancel the vibration-induced runout. To generate an appropriate cancellation signal, the disc drive must accurately measure the vibration of the disc drive using an internal accelerometer.
Since the positioning of the head requires a great deal of accuracy, it is preferred that the sensors be as accurate as possible. In addition, since space within the disc drive is limited, it is preferred that the sensor be as small as possible so that it does not take up valuable printed circuit board space. One type of sensor that is small and accurate is known as a Micro Electro-Mechanical System (MEMS). However, these sensors generate data at a rate that is incompatible with the lower data rate of the servo loop. Moreover, the rate at which these sensors produce data is generally fixed because the data rate is a by-product of the sensing characteristics of the sensor. Thus, the data rate of a MEMS sensor cannot be changed much once the other performance characteristics of the sensor have been chosen. As such, a sensor is needed that can be used with a servo system to generate a cancellation signal while providing the size and accuracy benefits of a MEMS sensor.